A first conventional magneto-optical disk head system has been disclosed in the text 29p-N-2 of Extended Abstracts (The 51th Autumn Meeting, 1990): The Japan Society of Applied Physics. The first conventional magneto-optical disk head system includes a semiconductor laser, an optical system including a collimator lens and an objective lens for focusing the laser light on a magneto-optical disk, a beam splitter, a half-wave plate, a polarizing beam splitter and a photodetector.
In operation, a laser light emitted from the semiconductor laser is transmitted through an optical system consisting of the beam splitter and the collimator and objective lenses to be focused on a magneto-optical disk. A reflection light of the laser light reflected at the magneto-optical disk reaches the beam splitter at which the reflection light changes its direction toward the half-wave plate. The half-wave plate rotates the polarization plane of the reflection light by 45 degrees to spatially separate the reflection light to P-polarized and S-polarized lights which have orthogonal polarization directions. The P-polarized and S-polarized lights are detected separately by the photodetector via the polarizing beam splitter.
Tile photodetector of the first conventional magneto-optical disk head system includes an N-layer grown by epitaxy on an N.sup.+ -silicon substrate, and two P-layers formed within the N-layer. The two P-layers compose two islanded segment regions for detecting the P-polarized and S-polarized lights, respectively. Each islanded segment region consists of several photodetecting segments.
Tile first conventional magneto-optical disk head system includes a signal detecting circuit for detecting an information signal and focusing and tracking error signals of the magneto-optical disk. In the signal detecting circuit, each of the photodetecting segments of the photodetector consists of a photodiode having a cathode corresponding to an electrode of a top surface and an anode corresponding to an electrode of a bottom surface. Cathodes of the photodetecting segments are connected in common to a bias power supply by which the signal detecting circuit is applied with a bias voltage. Low frequency components of signals supplied from the anodes of the photodetecting segments give the focusing and tracking error signals, respectively. On the other hand, high frequency components of signals supplied from the anodes thereof give the information signal by which data recorded on the magneto-optical disk are able to be playbacked.
second conventional magneto-optical disk head system has been disclosed in the text 30a-G-5 of Extended Abstracts (The 37th Autumn Meeting, 1990); The Japan Society of Applied Physics. The second conventional magneto-optical disk headsystem includes a semiconductor laser, a collimator lens, an objective lens, a beam splitter, a lens, a holographic element and a photodetector.
In operation, a laser light emitted from the semiconductor laser is transmitted through an optical system consisting of the collimator lens, the b earn splitter and the objective lens to be focused on a magneto-optical disk. A reflection light of the laser light reflected at the magneto-optical disk reaches the beam splitter at which the reflection light changes its direction toward the photodetector through the lens and the holographic element. The reflection light is divided by the holographic element to three lights, first one being a non-diffracted light (zeroth order diffracted light) whose polarization direction is orthogonal to an optical axis of a crystal composing the holographic element, and second and third ones being .+-.1st order diffracted lights whose polarization directions are parallel to the optical axis of the crystal. These three lights are detected by the photodetector.
The photodetector of the second conventional magneto-optical disk head system includes an N-layer grown by epitaxy on an N.sup.+ -silicon substrate, and three P-layers formed within the N-layer. The three P-layers compose three islanded segment regions for detecting the zeroth order and .+-.1st order diffracted lights, respectively. Each islanded segment region consists of several photodetecting segments.
The second conventional magneto-optical disk head system includes a signal detecting circuit for detecting an information signal and focusing and tracking error signals of the magneto-optical disk. In the signal detecting circuit, each of the photodetecting segments of the photodetector consists of a photodiode having a cathode corresponding to an electrode of a top surface and an anode corresponding to an electrode of a bottom surface. Cathodes of the photodetecting segments are connected in common to a bias power supply by which the signal detecting circuit is applied with a bias voltage. Low frequency components of signals supplied from anodes of the photodetecting segments give the focusing and tracking error signals, respectively. On the other hand, high frequency components of signals supplied from the anodes thereof give the information signal.
According to the first and second conventional magneto-optical disk head systems, however, there is a disadvantage in that the information signal and the focusing and tracking error signals are badly affected by each other by noises generated by the circuits for detecting these signals, because these three signals are detected through the common anodes of the photodetecting segments of the photodetector. As a result, the quality of the signals become deteriorated.